1. Field of the Invention
This invention relates generally to the area of synchronizing to data transmissions received by a data receiver. Particularly this invention relates to a signal detector and bit synchronizer for use in a portable paging receiver.
2. Description of the Prior Art
Bit synchronization to a digital transmission is a process used to determine the presence of symbol boundaries of a data transmission having symbols and thereafter to provide a clock to synchronously sample the symbols. When the signal modulation is a binary frequency shift keying (FSK) signal, each symbol corresponds to one bit. Other modulations may have other correspondence. Bit synchronization may be a process used in a paging receiver receiving and decoding a digital signalling protocol proposed by British Telecom in England which is commonly termed POCSAG (Post Office Code Standardization Advisory Group) which is a binary FSK modulation signal.
Synchronization to such a signal is known and has been described in detail in U.S. Pat. No. 4,518,961, May 21, 1985, to Davis et al. which shows synchronization to either the POCSAG or a Golay signalling protocols. Additionally, U.S. Pat. No. 4,506,262, issued Mar. 19, 1985 to Vance et al. shows synchronization to POCSAG using an early/late phase locked loop with coarse and fine synchronization modes.
Line 10 of FIG. 1 shows a typical POCSAG signal. Prior to the signal, noise or another type of protocol may be transmitted as shown in area 12 enclosed in a broken line. The POCSAG signal begins with a preamble signal, 14, which consists of a number of one-zero transitions. The preamble is followed by a plurality of thirty two bit information words, each coded in a 31,21 extended BCH code (32,21,1). The information words begin with a sync code word 16a which contains a predetermined binary sequence. At every seventeenth word thereafter another sync code, 16b, occurs in the signal. Between,, the sync codes the information is structured as eight information frames each of which contains two (32,21) words. For illustration, the contents of frame four, as indicated by the number 18 in FIG. 1, is shown on line 34. Line 34 has two thirty-two words, 36 and 38, each information word having thirty-two data bits structured in the (32,21) format. It can be appreciated that if the POCSAG protocol is ignored, the data bits shown on line 34 can appear to be effectively a random sequence of data transmitted at a predetermined baud rate.
The sync code provides a means for frame synchronization to the signal. Thus it is desirable to first bit synchronize to the preamble signal and subsequently frame synchronize to the sync code. Line 20 shows the operation of a pager synchronizing to the POCSAG signal. During interval 22 and 24, the pager is attempting to synchronize to the signal. However, the signal is not present. During interval 26, the preamble signal, 14, is present, the pager bit synchronizes and finds sync code 16a. Then in a known manner, the pager decodes information in preassigned frame 4 as shown by intervals 28 and 32. The pager also tests for sync code 16b during interval 30.
In some instances, the preamble signal may be corrupted by noise rendering the the preamble signal undetectable. In this situation, it is desirable to acquire bit synchronization on the data bits within the thirty-two bit words, and subsequently frame synchronize to one of the periodic sync code signals. The bit synchronization process in this mode is more difficult because the data in the thirty-two bit words is effectively random. Consequently, it is desirable to provide a selective call receiver capable of acquiring bit synchronization on either a POCSAG preamble signal or data signals within POCSAG information words.
Battery life is a critical aspect of portable selective call receivers and it is desirable to conserve battery power whenever possible. In the absence of the POCSAG signal, selective call receivers operate in a low power mode and periodically activate receiving and decoding circuitry in order to detect the presence of the POCSAG signal. If no signal is detected, the paging receiver again operates in a low power mode. This process conserves battery power.
Consequently, it is desirable to provide a means for detecting the absence of the POCSAG signal in a minimal time. In doing so, additional battery power may be conserved. Prior art pagers have typically analyzed a predetermined number of transitions and in response to various algorithms, determine the absence of the POCSAG signal., One such algorithm is shown in U.S. Pat. No. 4,554,665, issued Nov. 19, 1985 to Beesley. However, using a predetermined number of transitions requires waiting for the transitions to occur. Such techniques suffer greatly under conditions where transitions occur relatively infrequently, such as when low frequency tones are transmitted in place of the POCSAG signal. While waiting for all of the transitions to occur, the prior art receivers are consuming additional battery power Thus it is desirable to search for the POSCAG signal for a predetermined and substantially short time interval, while maintaining the ability to bit synchronize to the POCSAG preamble signal and/or the random data transitions. Rapid bit synchronization is also desirable in order to begin the frame synchronization process at the earliest possible moment. It is therefore desirable to gather transition data during a short integration time in order to either determine the absence of the POCSAG signal or establish a bit clock synchronized to the POCSAG signal.
Furthermore, prior art paging receivers typically establish a predetermined relationship between the sensitivity of detecting the POCSAG signal in a noise environment and falsely detecting of a POCSAG signal when only noise or another signal is present. However because paging receivers are used in many different paging environments around the world, a sensitivity and falsing performance in one application may not be optimal for another application. Thus it is desirable to construct a bit synchronizer for a paging receiver which can readily adapt sensitivity and falsing characteristics.
Finally, in order to conserve power in a paging receiver having a microcomputer, it is desirable to provide a bit synchronizer capable of operating at a reduced microcomputer bus speed. Prior art paging receivers with microcomputers typically sample the incoming signal at a very high rate, and typically use a digital phase locked loop implemented in software in order to establish a bit clock for sampling data bits after synchronization. Software generated digital phase locked loops require high sampling rates and continuous phase adjustments in a real time software environment. This requires a microcomputer to operate at a relative high data rate.
U.S. Pat. No. 4,414,676, issued Nov. 8, 1983 to Kraul et. al. shows a synchronizer which, in the preferred embodiment, samples at five times the data rate and performs numerous calculations between each sample. Kraul et. al. does not show the capability to synchronize on random data. Thus it is desirable to construct a bit synchronizer which provides for a low sample rate. It is desirable to construct a bit synchronize which in one operation establishes a bit clock synchronized to the data signal. These operations will provide for operating the microcomputer at a reduced bus rate thereby reducing the power consumption and extending the battery life of the paging receiver.